This invention relates to glass compositions for seals, to sealing compositions including the glass composition and fillers, to methods of manufacturing such compositions, to methods of manufacturing a seal, to optical components having such seals, and to methods of offering a data transmission service over a fiber having such a seal.
Forming a good seal with a strong bond between an optical fiber and an optical component is important as many optical components are subject to harsh operating conditions including vibration, high humidity and temperature cycling. Several problems are associated with the bonding process between a glass material and a non-glass material generally, and the high design specifications for optical components exacerbate these problems.
For example, temperature variations require any bond formed ideally to match the thermal coefficients of expansion of the optical fiber and the bonded part to mitigate thermal stress on the optical fiber. Determining the composition of a glass fixative having a sufficiently low melting point to enable an optical fiber to be bonded to a non-glass material without deforming the optical fiber, having a desired thermal coefficient of expansion, and good adhesive properties to both the silica of the optical fiber and the non-glass material is a difficult and complex task. Alternative processes using solder compounds such as Sn/Pb alloys were employed instead.
Most optical components include parts which have a non-glass composition, for example, metallic materials such as Kovar. One known method of bonding glass to such metallic materials requires the glass fiber to be metalised. The metalisation process required a fiber to be stripped to its core and given a metallic coating consisting of a bonding layer and a soldering layer. This enables bonds between the metalised glass fiber and the Kovar material to be soldered.
Metalisation processes have several disadvantages. The fibers have to have their adhesion verified and any masking material used must be removed. Such metalisation processes are time consuming and the fiber strength can be significantly reduced as a result (typically for example by 30%). Other disadvantages include the extensive handling of fibers required by such processes and the associated high fiber breakage rate, and the capital expenditure on plant required by such processes. The preparation of the fibers for metalisation and soldering is moreover time-consuming Yet another disadvantage of fiber fixing using metalisation processes is that no reworking is possible during either the metalisation or soldering stages. A further disadvantage of metalisation processes for fiber fixing is that the soldering process can leave behind potentially corrosive fluxes. The soldering process usually involves the use of acidic, organic fluxes and the organic residues can adversely affect laser or optical performance if not cleaned off effectively.
The cleaning process itself can cause concerns for product performance which need addressing. These processes also have the effect of weakening the fiber because of the pre-cleaning required and because of stress concentrators associated with the metalisation. The softer solders can suffer creep and hence be detrimental to alignment stability. In complex optical components, a further problem is the necessity of preventing preexisting bonds from being degraded when subsequent bonds are formed in the near vicinity.
Alternatives have also been developed such as resin fixing-uvlight and/or heat curing. However, the resins have poor high temperature performance and lengthy cure cycles with messy dispensing and out gassing during processing.
The bonds must often be sufficiently strong and intact to form a hermetic seal between the optical fiber and the corresponding portion of the optical component to isolate the interior of the optical component from the external atmosphere. This enables the atmosphere within the optical component to be isolated and for non-air atmospheres or pure air atmospheres to be used. Moreover, the moisture content of the interior can then be controlled. It is thus important for any bond formed to be sufficiently strong to retain the hermeticity of the seal when subject to thermal stress and/or vibration and/or shock.
An example of a typical requirement is to provide a means for passing an optical fiber hermetically ( less than 1xc3x9710xe2x88x929 Atmos cc/s) through the wall of a package. At the same time the fiber should not be weakened and the optical performance of the fiber should be unimpaired. Another challenge is to provide a quick, simple and reliable means of fixing a fiber position very accurately (sometimes to less than 0.1 um) subsequent to alignment and keeping it there for the lifetime of the product. The fixing method should be clean in itself and the resultant assembly should be chemically, mechanically and optically stable.
One current technique for sealing fiber tails to optical components without metallising the fibers involves using a low melting point glass to make a hermetic seal between the fiber and a Kovar hypo tube. To carry out this operation a glass preform is heated to 450xc2x0 C. approximately in an induction coil. The nominal sintering temperature to manufacture the glass preform is 310xc2x0 C.
An example of a commercially available low melting point (LMP) glass usable for the hermetic seal, developed by NEG (Nippon Electric Glass), is shown in U.S. Pat. No. 5,346,863. It involves a PbO B2O3 type glass matrix, mixed with a ceramic filler powder to reduce the temperature coefficient of expansion (TCE) to match the glass and metal materials being bonded. Due to environmental pressures there is a need for alternative or better glasses,
It is an object of the present invention to provide improved apparatus and methods. According to a first aspect of the present invention, there is provided a sealing glass composition consisting essentially of: 70-75 wt % of PbO, 3-7 wt % of PbF2, 5-8 wt % of Bi2O3, 5-7 wt % of B203, 2-5 wt % of ZnO, 1-7 wt % of Fe2O3, 0-2 wt % of CuO, 0-3% of TeO2, and a trace  less than 0.2% of MnO2, the composition having a flow temperature of  less than 350xc2x0 C.
An advantage of this is that it can be used to make seals which can be flowed at low temperatures, using different constituents to those used in prior compositions. This helps reduce the risk of damage to temperature sensitive materials near the seals, for example polymer coatings of optical fibers. Also, this composition can achieve such low flow temperatures without excessive degradation of other useful properties such as low viscosity, low TCE, good adhesion to glass and metals, low permeability of air, good long term hydrolytic stability, and so on. It can be mixed with a filler such as a ceramic powder, to enable the TCE to be matched accurately to the materials being sealed.
It can be used to fix silica fiber into electro-optic devices to achieve hermetic joints and to give high mechanical stability fixing of the fiber and other optical train related components for example. It can be heated to flow to form a seal using a Nd/YAG, CO2 or diode laser, hot plate, hot air and induction heating. Of these the preferred heat sources are induction heating and the Nd/YAG laser. It can give substantial cost reduction, performance improvement and new design options whilst being quick, clean, repeatable and stable. It has many possible applications. The process and materials are much more environmentally friendly than prior processes and materials. Material delivery can be in various forms including powder, preform and paste (allowing screen printing options).
As a preferred feature for a dependent claim, the temperature coefficient of expansion is between 4 and 12 ppm/xe2x88x92C. This is another key property to ensure long term reliability of the seal in realistic conditions of temperature changes. This TCE is the value before matching by adding filler. It is useful to keep this value low so that the proportion of filler can be kept low, since the filler usually increases the flow temperature significantly, for example by 10xc2x0 C. for each 1% of filler.
As another preferred feature for a dependent claim, the glass composition has one or more of V2O5, P2O5, TeO2, SnO2, Co3O4, Li2O As2O3, Sb2O3 
This corresponds approximately to table 1 below.
As another preferred feature for a dependent claim, the glass composition, having 5 to 15 wt % B2O3, 5 to 16 wt % Bi2O3, 5 to 9 wt % ZnO, 2 to 8 wt % F, and 2 to 5 wt % TeO2.
This corresponds approximately to mix T-3 of table 3 below.
As another preferred feature for a dependent claim, the glass composition has 6 to 7 wt % B2O3, 7 wt % Bi2O3, 3 to 4 wt % ZnO, 5 to 6 wt % F, 1 to 2 wt % Teo2, 0.15 wt % MnO2, 2 to 3 wt % Fe2O3 and 1 to 2 wt 4 CuO.
This corresponds to mix xe2x80x9cCxe2x80x9d of table 5 below.
Another aspect of the invention provides a composition comprising 70-99 wt % of the glass composition, and 1-30 wt % of a ceramic filler, the composition having a TCE of 1 to 5 ppm/xc2x0 C. and a flow temperature of less than 400xc2x0 C.
As a preferred feature for a dependent claim, the composition has sufficient adhesion to glass and to metal to withstand a bend test of 3 mm deflection, 2 mm from the seal.
As another preferred feature for a dependent claim, the composition has a durability sufficient to maintain seal hermeticity  less than 1xc3x9710xe2x88x929 Atmos cc/s without optical power losses of packaged devices after high humidity exposures after 2000 hrs at 85xc2x0 C. and 85%RH, according to standard test procedures such as MIL-STD-883 Method 103.
As another preferred feature for a dependent claim, the composition has a hermetic seal level of  less than 1xc3x9710xe2x88x924 Atmos cc/s.
Another aspect of the invention provides a seating glass composition consisting essentially of:
70-75 wt % of PbO,
3-7 wt % of PbF2,
5-8 Wt % of Bi2O3 
5-7 Wt % of B203 
2-5 wt % of ZnO
1-3 wt % of Fe2O3 
0-2 wt % of CuO
0-2% of TeO2
and a trace  less than 0.2% of MnO2 
Another aspect of the invention provides a method of manufacturing the composition comprising the steps of: mixing the PbO, and metal oxide components, reacting the mixture with ammonium bifluoride to create PbF2, and adding a low TCE ceramic filler.
Another aspect of the invention provides a method of manufacturing a seal using the composition, comprising the steps of placing a preform of the composition over a gap to be sealed, and applying localized heating to cause the preform to flow into the gap.
As another preferred feature for a dependent claim, the localized heating can be induction heating or laser heating.
Another aspect of the invention provides an optical component having a seal having the above mentioned composition.
Another aspect of the invention provides a method of offering a data transmission service over an optical network along an optical fiber sealed using the composition As the advantages of the invention can enable a better network, which is more reliable or more cost effective for example, consequently a data transmission service over the network can show a corresponding improvement, and the value of such services can increase. Such increased value over the life of the system, could prove far greater than the sales value of the equipment.
Any of the features can be combined with any of the aspects of the invention as would be apparent to those skilled in the art. Other advantages will be apparent to those skilled in the art.